To detect a test substance in a sample liquid, many conventional methods of using an antibody that can specifically bind with a test substance are devised. One of such methods is to optically detect immune aggregate generated by an antigen-antibody reaction.
With this method, first, a predetermined amount of an antibody that can specifically bind with a test substance is added to a sample liquid containing the test substance, to form aggregate. Next, upon radiating light to the sample liquid containing the aggregate, the light is scattered by hitting the aggregate, so that the amount of light passing through the sample liquid changes. The change of the amount of light is known by measuring either transmitted light, reflected light or scattered light.
Generally, although a turbidimetric immunoassay which involves measuring transmitted light and a nephelometric immunoassay which involves measuring scattered light are distinguished, these two measurement methods have the same detection principle of observing the influence of light scattering by aggregate. The amount of aggregate changes in proportion to the concentration of a test substance, so that, it is possible to find the concentration of a test substance by measuring either transmitted light, reflected light or scattered light.
A method of making these methods more sensitive is the latex agglutination assay. In the latex agglutination assay, antibodies are immobilized on latex particles. When a test substance is bound with the antibodies, aggregate of latex particles is formed. The aggregate of latex particles allows light to scatter much more than aggregate consisting of antigens and antibodies, so that highly sensitive detection is possible.
Patent Document 1 discloses the method of making the latex agglutination assay more sensitive. The measurement apparatus and the measurement method disclosed in Patent Document 1 will be explained with reference to FIGS. 1 to 4. FIG. 1 is a cross-sectional view of a reaction chip, FIG. 2 is a block diagram of a reaction measurement apparatus, and FIGS. 3 and 4 are exploded views of electrode parts of a reaction chip. Electrodes 3 and 4 having a thickness of 0.02 mm are interposed between slide glasses 1 and 2 so that the distance between electrodes 3 and 4 is 0.5 mm. Latex particles 20 are dispersed in a sample liquid in flow path 14 surrounded by slide glasses 1 and 2 and electrodes 3 and 4. By applying an alternating voltage to electrodes 3 and 4 using alternating power source supplying apparatus 5 and applying an electric field to the above reaction system, a phenomenon referred to as “pearl chaining” (19) where latex particles 20 are aligned linearly occurs.
After that, when application of the electric field is stopped, latex particles 20 that have been aligned linearly disperse again. If the test substance (antigen) is present upon pearl chaining, latex particles 20 do not disperse again even after application of the electric field is stopped, and pearl-chained particles still exist. Consequently, by measuring the state of aggregate of latex particles 20 not dispersing again even after application of an electric field is stopped, that is, by measuring the state of aggregate of latex particles aggregating by an antigen-antibody reaction using an image process apparatus configured with microscope 7, CCD camera 8, image processing board 9 and personal computer 10, it is possible to detect or measure a test substance (antigen).
Patent Document 1: Japanese Patent Application Laid-Open No. 07-83928